The invention relates to a process for the removal of sour gases from pressurised natural gas which is polluted by sulphur compounds and other sour gas compounds. The process can be used for natural gas types that contain not only useful substances, such as methane and higher hydrocarbons but also impurities, such as hydrogen sulphide, organic sulphur components, e.g. mercaptanes, and carbon oxysulphide, as well as carbon dioxide and water vapour in different portions, the process according to the present invention being particularly suitable for natural gas types with a high hydrogen sulphide content.
As a rule, the sulphur components contained in the raw natural gas have to be removed to obtain a residual content of only a few ppm to permit further industrial utilisation of the natural gas. The removal of hydrogen sulphide, mercaptanes, carbon dioxide and other sour gas constituents from industrial gases is generally performed with the aid of chemically acting absorbents, such as amino solutions, alkali salt solutions, etc. or physically acting absorbents, such as Selexol, propylene carbonate, N-methyl pyrrolidone, Morphysorb, methanol, etc. in loop systems, the physically acting absorbents (as opposed to chemical scrubbing agents) being capable of removing organic sulphur components. In this process, the carbon dioxide contained in the gas is removed partially, totally or only as little as possible, depending on the requirements and specifications. An appropriate state-of-the-art technology is, for instance, described in DE 197 53 903 C2.
Normally, the sour gas from the absorbent regeneration unit is further processed to sulphur in a Claus plant. This means that, in addition to the investment costs for the desulphurisation unit itself, the investment costs for a Claus plant must also be taken into consideration for the overall investment costs. Anti-pollution legislation governing the residual content of sulphur components in a Claus plant necessitates the provision of a so-called “tail gas treatment unit” for the final desulphurisation of the Claus waste gas, which in fact significantly increases the investment costs further. Because of the world-wide surplus of elemental sulphur, which in turn originates from the desulphurisation of gases containing hydrogen sulphide, the recovered sulphur will only yield a negligible sales profit that would otherwise contribute towards the amortisation of the investment costs.
As an alternative to the recovery of elemental sulphur, therefore, the re-injection and storage of the sour gases obtained during the regeneration of absorbents in gas caverns is ever more often considered. In this process, the sour gases are compressed by means of costly compressors to a pressure that permits the sour gases to be pumped into an underground gas storage cavern, such as an exhausted gas field. The final pressures required for such re-injection are normally higher than the pressure at which the sulphur components are removed.
It would be particularly advantageous for such purposes if the sour gases obtained during the regeneration were at as high a pressure level as possible, because the investment costs could then be considerably reduced as a result of the required compressors being smaller or even obviated, and also because savings in the compressor operating costs can be achieved. However, according to the current state of engineering technology, chemical or physical absorption processes are used for the absorption of hydrogen sulphide, mercaptanes, carbon dioxide and other sour gas constituents and for the preparation of the concentrated sour gases for re-injection into an underground storage cavern. In both processes, the regeneration of the absorbent takes place at a slightly elevated pressure of 0.1 to 1 bar(g). The respective sour gas is also obtained at this low working pressure.
The aim of the invention is thus to provide a process that is equally suitable for the treatment of polluted natural gas and for providing sour gas at an elevated pressure, and which provides the sour gas in a state that is suitable for its re-injection into gas fields that are or are about to be exploited.